This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have been studying the molecular mechanism of the main phase transition (the gel to liquid crystalline phase transition) of lipid bilayers. Previously, we observed the quasicritical fluctuation in DOPC bilayers around Tc. During the past year, we improved the simulations of ESR spectra of the head group spin label DPP-Tempo and chain spin labels 5PC and 14PC in DMPC bilayers around Tc at 23C. Two components are required for a satisfactory fit of the spectra from 5PC and 14PC, which is consistent with previous ESR and NMR studies on the main phase transition of lipid bilayers. When the temperature of a DMPC dispersion is cooled down from 30C to 15C, changes of the dynamic ordering of the two components are quite different. Order parameters S0 of the second component of 5PC and 14PC spectra show a strong critical phenomenon around the Tc. For example, S0 of 5PC sharply drops from 0.591at 24.0C to -0.457 at 23.3C, then jumps to 0.903 at 22C. These changes are characteristic of a second order phase transition. In contrast, there is nearly no change or a slight change in S0 around Tc for first component. We also observed a divergence in the linewidth of 5PC and 14PC spectra around the Tc, which is also a sign of a second order phase transition. Unlike the spectra of 5PC, the spectra of DPP-Tempo in DMPC can be fit very well with only a single component, and the variation of the S0 with temperature exhibits a cusp-like pattern around Tc, similar to that we previously observed for DPP-Tempo in DOPC around Tc. This is also a critical phenomenon. Interestingly, the tip of the cusp is located at 24.4C, which slightly deviates from Tc. The rotational diffusion rates R perpendicular and R parallel of the second component for both 5PC and 14PC drop precipitously at 23.3C (by factors of more than 100). The population of the first (second) component decreases (increases) with decreasing temperature only gradually below and above the Tc, but drops (jumps) around the Tc from ~0.8(~ 0:2) to 0.5(~0.5). According to IR studies of the main phase transition of lipid bilayers, the two populations may represent lipids which have two different acyl chain conformations, thus they have two different packing modes in the bilayers. This type of temperature variation of two lipid populations signifies a first order phase transition at Tc. However, overall the main phase transition of DMPC bilayers is a weak first order phase transition, because of concomitant strong critical phenomena.